<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    for(var i=1;i<=3;i++){
        setTimeout(function () {
            console.log(i);
        },0)
    }
    // 输出为444

    for(let i=1;i<=3;i++){
        setTimeout(function () {
            console.log(i);
        },0)
    }
    // 输出为123

    var a = 1;
    function foo() {
        if(!a){
            var a = 2;
        }
        alert(a);
    }
    foo();
    // 输出为2
</script>
</html>